1. FIELD OF THE INVENTION
This invention relates to composite structural materials, and especially to sliding and friction elements in which a functional layer, such as a coating containing tin and/or copper, constitutes a friction or slide surface layer, such layer being applied to a substrate of steel with an intermediate layer that contains copper and/or tin and which has a diffusion barrier coating with a thickness of 0.5 to 5 um.
2. DESCRIPTION OF THE RELATED ART INCLUDING INFORMATION DISCLOSED UNDER 37 CFR .sctn..sctn.1.97-1.99
In known composite structural materials, such as those according to German Patent Publication DE-OS No. 28 53 724 there is provided an intermediate layer, such as of lead bronze, and on such intermediate layer a functional layer, such as a slide or friction coating. A thin oxide-free diffusion barrier layer is disposed between the intermediate and friction layers in order to prevent tin from diffusing from the friction layer into the intermediate layer at elevated temperatures. This is intended to counteract the formation of intermetallic brittle phases of tin and copper in those areas of the intermediate layer which are located in the vicinity of the slide and friction coating. Such brittle phases can cause detachment of the slide and friction layer under conditions of dynamic stress. The DE-OS Patent Publication No. 28 53 724 therefore proposes to provide a sputtered diffusion barrier layer of NiCr.sub.20 or chromium between the intermediate layer and the slide or friction layer.
Practical experience has shown that the nickel-chromium diffusion barrier layer and also a pure chromium diffusion barrier layer is not fully effective in tin-containing slide or friction layers (e.g. from AlSn alloys). Studies on the electron microscope and the microprobe show that tin evidently diffuses into and over lattice void spaces through a diffusion barrier layer of NiCr.sub.20 or pure chromium which had been prepared by cathode sputtering, such diffused tin combining with the copper of the lead bronze intermediate layer to create an intermetallic brittle phase, such as Cu.sub.6 Sn.sub.5, which under conditions of practical operation will lead to detachment of the slide layer.
Conventional galvanically applied diffusion barrier coatings of all known material compositions have been proved ineffectual at elevated operating temperatures since they have a relatively large number of lattice void spaces and therefore allow considerable amounts of tin to diffuse from the functional slide or friction layers, into the intermediate layer. Conventional diffusion barrier layers were found to be inadequate, due to their characteristic substantially higher permeability for tin.
In addition to the example given above, a diffusion barrier layer should also prevent the diffusion of tin from a tin-containing intermediate layer into a copper containing functional layer. The prevention of tin diffusion in one or the other direction is of increasing importance in that machine elements need to be developed for higher operating temperaturs because of the trend to increase the machine efficiency, particularly in combustion engines.